In this arrangement, data exchange or more specifically the exchange of process variables takes place on the basis of OPC. OPC is a standardized communications protocol which has been specified by the OPC Foundation (www.opcfoundation.org). OPC stands for OLE (“Object linking and embedding”) for Process Control and is an open standard.
Process automation systems are normally subdivided into three hierarchically arranged automation levels. In the upper automation level, which is often termed the management layer or supervision layer, personal computers are mainly used. The tasks at this level are, for example, operator control, monitoring, display, archiving and optimization of the process operations. In the control layer below this level, because of their extremely high availability, so-called programmable logic controllers (PLCs) are preferably used which, as they mainly only ever control one sub-process, are also known as distributed automation units. The PLCs acquire the process measurements acquired by sensors via corresponding connections, the sensors acquiring measurements directly from the relevant sub-process, e.g. from a turbine or electric generator, in what is known as the field layer. The process measurements acquired by the sensors can also be forwarded by microcontrollers disposed in the field layer to the relevant PLC or PLCs or can be constituted by the microcontrollers themselves. In the same way, the PLCs can address the actuators disposed in the field layer via control signals and possibly via the microcontrollers in order to influence the sub-processes, e.g. the turbine.
Automation devices such as programmable logic controllers of the kind present in the control layer, and personal computers of the kind present e.g. in the management layer of a large power plant, communicate with and among one another mainly by means of proprietary communications protocols. These communications protocols mainly differ not only between different manufacturers, but often also between devices of the same manufacturer. This makes communication with other automation devices impossible without costly measures. Thus it is possible only at great expense to upgrade, modify or modernize an existing automated plant of this kind using other manufacturers' automation devices or software programs.
In order to standardize the communications interfaces between the different devices and software programs and thereby standardize communication, the OPC interface has been developed and agreed as a standard in the process automation sector. Nowadays OPC-based communication is mainly used between the management layer and the control layer. OPC-based communication of this kind can also be used between a business/office level and the management layer or within the same layers. Data or more specifically process variables are exchanged or transmitted via OPC connections.
The data exchange of an OPC communication generally takes place via n active connections, an OPC-based communication requiring ever higher communication availability with correspondingly shorter downtimes or failure frequencies. For this reason, a reliable OPC connection is also desirable for an OPC-based communication.
In order to make an OPC connection more reliable, it is known from the prior art, for example, to duplicate the hardware, which means that two complete hardware connections are provided separately from one another. In the event of a fault, switching then takes place to the fault-free hardware connection. However, this solution does not take operator control into account. As a result, after a switchover the displays (plant displays and/or operating displays) must be re-selected. In critical plant states, valuable time is lost, so that dangerous operator control breaks can occur. A high-availability OPC connection cannot therefore be implemented solely by connecting OPC client-server connections i.e. OPC links in parallel, as continuous operator control cannot be guaranteed in the event of OPC link failure. Thus operating displays of the HMIs (Human-Machine Interfaces) frequently disposed in the control layer may be disturbed by the switchover and therefore have to be updated in a time-consuming manner by re-interrogation. Operator inputs and control commands may likewise be lost. Particularly in the case of time-critical operations, this is a considerable disadvantage which can result in dangerous plant states. As a result, multiple archives must also be maintained, as complete acquisition and archiving cannot be guaranteed during the switchover phase. Another disadvantage is that process states and process alarms must be updated in the switchover phase or during re-integration. Events occurring in the meantime (e.g. changes and alarms) may be lost as a result.
In order to achieve such continuous and uninterrupted operator control and monitoring of the automation level from the management layer even in the event of computer failure, exclusively proprietary solutions have hitherto been used which. e.g. in the case of modifications, allow only little scope for selecting new devices or must be adapted at very great expense.